1. Field of the Invention
The present invention relates to a self-sensitive polymerizable liquid resin, which provides excellent properties in deep curing and dark-color curing. The present invention also relates to a coating composition containing the resin, which can be coated on a surface of a substrate to form a film.
2. Description of the Related Art
Environmental concerns have prompted widespread adoption of radiation curing processes such as UV curing in the preparation of coatings. Due to improvements in raw materials, a UV-cured coating can be prepared at commercial production speed by addition of a photo initiator, so as to produce printed matter or film with good haze, durability and chemical resistance. However, the conventional photo initiator (such as benzophenone) has disadvantages in terms of toxicity, cost, and odor. The conventional photo initiator also has the disadvantage of easily resulting in Newton's rings, which limits the practical value of the photo initiator on transparent, white and light-colored coatings.
To form a thick coating on plastic or glass, a dual-curing process combining thermo curing and photo curing can be adopted, in which a mixture of different photo and thermo initiators are used to ensure the curing of the surface and the interior of the coated thick film (such as a film produced by screen printing), so as to form a functional film. However, such film is expensive to produce, involves numerous restrictions on selection of reagents and increases the complexity of manufacturing, so the process is economically disadvantageous.
Presently, efforts are underway to reduce the amount of the photo initiator in a photo curing coating formulation by blending a self-sensitive polymerizable resin and a color material with the coating composition. The optical activity inherent to such radiation curable resin allows a good UV curing effect to be achieved without using a photo initiator.
The self-sensitive polymerizable resin can be a multifunctional acrylate resin formed by reacting an acrylate monomer and oligomer with a compound that can participate in a Michael addition reaction, for example, β-keto ester (such as acetoacetic ester), β-diketone (such as pentane-2,4-dione), β-keto amide (such as acetoacetanilide and acetoacetamide) and/or other β-dicarbonyl compounds.
Compared with a common formulation containing a photo initiator, the formulation containing the self-sensitive polymerizable resin has better odor characteristics, because no photo initiator remains after curing. Common photo-polymerization monomers are crosslinked after ultraviolet irradiation in the presence of a photo initiator, or cured due to triggering of free radicals that are activated by a peroxide. The photo initiator and/or peroxide is a typical low-molecule multifunctional compound, can volatilize and generates an undesired odor after environmental quenching, and can be easily absorbed by skin, thus having adverse impact on health. Use of a functional oligomeric photo initiator can overcome some of the disadvantages, because a polymerized photo initiator is generally a non-volatile compound, and is not easily absorbed by skin. However, preparation of the functional oligomeric photo initiator includes multiple synthesis steps, and the functionality is unfavorable to the reaction and the final desired property. When a self-sensitive polymerizable resin is used, no odor is generated or volatilized even if some resin remains, since the resin is a non-volatile polymer.
However, there are many restrictions on synthesis of a conventional self-sensitive polymerizable resin. For example, Mozner and Rheinberger (Macromolecular Rapid Communications, volume 16 p 135-138, 1995) report that an acetoacetic ester donor compound can be added to a multifunctional acrylate acceptor compound to undergo a Michael addition reaction. However, if the acetoacetic ester is added into an acrylate compound having three or more functional groups through an addition reaction, the crosslinked polymerization product is crosslinked gel. Although the product still has unreacted acryloyl functional groups, the generated network structure can be regarded as a gelatinized or cured structure. Since the network structure has been substantially crosslinked, it cannot be turned into a liquid state by heating or dissolving it with a solvent. Therefore, there is an urgent need in the industry to resolve the issue of how to use a multifunctional Michael acceptor without gelation.